Technical Field:
The disclosure concerns a plasma reactor for plasma processing of a workpiece, in which the plasma source employs an electron beam.
Background Discussion:
A plasma reactor for processing a workpiece can employ an electron beam to generate plasma in a processing chamber. The electron beam source produces a high-energy, sheet electron beam, which is injected into the processing chamber. As the electron beam propagates through the processing chamber, it produces plasma for etching or other applications.
In electron beam plasma processing of workpieces such as semiconductor wafers, an electron source produces a sheet electron beam, which is then injected into a processing chamber. As the electron beam propagates through the chamber, it produces a plasma having low plasma electron temperature and a low plasma ion energy. Such a plasma is useful for etching, cleaning, and other applications. However, due to the low electron temperature of the plasma electrons and small dissociation cross-section for many molecules at beam electron energies (˜1000-3000 eV), the dissociation of many gas molecules, e.g., Cl2 or NF3 or SF6, is not sufficient. For some critical applications, i.e. silicon etch, which require significant amount of Cl or F radicals, another means of radical generation is needed.
Another problem is metal contamination in the plasma. A direct-current (dc) electron beam plasma source must include exposed metal surfaces, which provide current continuity required for steady-state operation. Some of these surfaces typically experience bombardment by energetic ions, and may include, for example: (1) the interior surfaces of the electron source discharge chamber, where electrons are generated before being extracted to form the beam; and (2) the surface of the extraction grid facing the processing chamber. In the source discharge chamber, there is a cathode sheath at the surface with a substantial voltage drop (e.g., about 250 eV), in which ions are accelerated towards the metal interior surface. In turn, ions that diffuse into the acceleration gap are picked up by the electric field that accelerates electrons of the beam towards the processing chamber. As a result, these ions are accelerated to significant energies (e.g., on the order of 2 keV) towards the extraction grid. In both the discharge chamber and the acceleration gap, these fast ions bombard the metal surfaces and cause sputtering, so that sputtered metal atoms emerge from the electron beam source and propagate with the electron beam. This leads to metal contamination of the processing chamber walls and the workpiece or substrate being processed. Although the ion energy is much higher in the acceleration gap, the ion flux density is probably much higher in the discharge chamber, so sputtering of metal atoms from the discharge chamber surface can also be substantial. There is a need to prevent the sputtered metal atoms from reaching the processing chamber.